As a method for molding a film or a sheet, for example, there is known a pressure molding method in which molten resin pushed out from a T die is inserted and pressed (which is hereinafter expressed as compressed) by a pair of molding rolls with high rigidity, and is molded into a sheet-like shape and is cooled simultaneously.
As a typical apparatus for embodying the above pressure molding method, as shown in FIG. 11A, there is available a sheet molding apparatus 100 structured such that three molding rolls 101, 102 and 103 are disposed parallel to each other in the horizontal direction (see, for example, JP-A-2011-116027 and Japanese Patent No. 3194904).
The sheet molding apparatus 100 includes a touch molding roll 101, a main molding roll 102 and a finish molding roll 103 sequentially parallel disposed from the front-stage side toward the rear-stage side in the horizontal direction, in which a molten sheet 105 is supplied downward from a T die 106 existing above to a nip part 104 interposed between the touch molding roll 101 and main molding roll 102, is compressed by the touch molding roll 101 and main molding roll 102, and the compressed sheet 105 is wound and fed from the main molding roll 102 to the finish molding roll 103, thereby molding the sheet 105 having a specific thickness.
Here, the nip part 104 means a roll gap in which the sheet 105 is held and compressed by the two adjacent molding rolls 101 and 102.
In the sheet molding apparatus 100, between the touch molding roll 101 and main molding roll 102, the outer peripheral surfaces of the touch molding roll 101 and main molding roll 102 are simultaneously contacted with both sides of the sheet 105. Specifically, through the contact with the touch molding roll 101, the sheet 105 is cooled just instantly, whereas most of cooling of the sheet 105 is attained by the main molding roll 102.
The sheet winding angle of the finish molding roll 103 is normally 90° and the cooling of the sheet 105 ends at a position where the sheet 105 is parted from the finish molding roll 105.
A thick sheet requires a long cooling time and thus is normally molded at a low speed, whereas a thin sheet requires a short cooling time and thus is necessarily molded at a high speed.
Referring to the number of cooling times on the front surface (upper surface) and back surface (lower surface) of the sheet 105 by the three molding rolls 101 to 103, the back surface of the sheet 105 is cooled two times, whereas the front surface is cooled one time.
The diameters of the three molding rolls 101 to 103 are all set equal; or, the diameters of the main molding roll 102 (the second roll) and finish molding roll 103 (the third roll) are set large, while the touch molding roll 101 (the first roll) is normally set small.
The reason why the diameters of the main molding roll 102 and finish molding roll 103 with great sheet winding angles are set large is that the cooling heat required is directly proportional to the sheet contact length.
Also, as a related-art sheet molding apparatus using four or more molding rolls, for example, there are known apparatus which are disclosed in JP-A-H10-264194 and U.S. Pat. No. 8,262,966.
As shown in FIG. 11B, a sheet molding apparatus 110 disclosed in the JP-A-H10-264194 includes: a first roll 111 and a second roll 112 disposed parallel to each other on front and rear stage sides in the horizontal direction; and, a third roll 113, a fourth roll 114 and a fifth roll 115 sequentially disposed parallel to each other downwardly of the second roll 112, in which a molding material 116 formed of thermoplastic resin such as vinyl chloride resin (PVC) or rubber lumps supplied between the first and second rolls 111 and 112 is kneaded and molten with high line pressure by the first and second rolls 111 and 112, and the molten molding material 116 is rolled by the third to fifth rolls 113 to 115 disposed downwardly of the second roll 112, thereby molding a sheet 117.
Here, the line pressure means a force acting on a unit area of 1 cm in the roll longitudinal direction when a pair of rolls are pressed against each other (for example, 98N/cm (10 Kg/cm)); and, the line pressure is also called nip pressure.
As shown in FIG. 11C, a sheet molding apparatus 120 according to the U.S. Pat. No. 8,262,966 includes: a first roll 121 and a second roll 122 disposed in parallel to each other in the vertical direction; and, a large number of small-diameter rolls 123 which are disposed parallel to the first roll 121 on the upper stage side in the horizontal direction and the heights of which are different from each other alternately in the vertical direction, in which a molding material 125 is supplied in a sheet-shaped manner to a nip part 124 intervening between the first and second rolls 121 and 122 from the lateral direction, is compressed by the first and second rolls 121 and 122, and the compressed sheet-shaped molding material 125 is sequentially wound on and fed by the large number of small-diameter rolls 123, thereby molding a sheet.
However, the related-art sheet molding apparatus 100 shown in FIG. 11A has the following problems.
That is, since the number of molding rolls 101, 102, 103 used to mold the sheet 105 is three and the number of nip parts 104, 107 for holding and pressurizing the sheet 105 by the mutually adjacent molding rolls 101, 102 is two, there cannot be secured a sufficient contact cooling distance for cooling the molding rolls 101, 102, 103 and sheet 105 in a state where they are closely contacted with each other without intervening an air layer between them.
Also, with respect to the number of cooling times on the sheet 105 by the three molding rolls 101, 102, 103, the front surface (upper surface) side of the sheet 105 is cooled one time and the back surface (lower surface) side is cooled two times, that is, the number of cooling repetition times on both sides of the sheet 105 is small. Also, when compared with the roll contact time between the main molding roll 102 and sheet 105, the roll contact time between the touch molding roll 101 and sheet 105 is extremely short; the roll contact time between the finish molding roll 103 and sheet 105 is about half of the roll contact time between the main molding roll 102 and sheet 105; and, with respect to the total roll contact time between the main molding roll 102 and sheet 105, the time on the sheet front side is longer than the time on the sheet back side. Thus, the high temperature part of the sheet 105 is unevenly distributed in the sheet back side surface distant from the contact surface with the main molding roll 102, thereby worsening cooling efficiency.
Therefore, the related-art sheet molding apparatus 100 lacks cooling capacity in total and thus is incapable of molding a thick sheet slow to cool (for example, a sheet having a thickness dimension t=0.6 mm or larger) at high speeds and with good quality.
Also, since a thin sheet (for example, a sheet having a thickness dimension t=0.1 to 0.6 mm or smaller) is quick to cool, it is necessarily molded at high speeds. However, since there are provided only the two nip parts 104 and 107, the holding/pressing operation in the nip parts 104, 107 is executed only two times in total, thereby degrading the quality such as gloss and transparency of the sheet surface. Thus, the thin sheet quick to cool cannot be molded at high speeds and with high quality.
And, the related-art sheet molding apparatus 110 shown in FIG. 11B is structured such that the molding material 116 is kneaded and molten by the first roll 111 and second roll 112 with high line pressure and the molten molding material 116 is rolled by the third to fifth rolls 113 to 115 disposed downwardly of the second roll 112.
Therefore, this apparatus 110 is different in the basic structure from the related-art sheet molding apparatus 100 shown in FIG. 11A which relates to the premise technique of the invention and is structured such that, after previously kneaded and molten, the molten sheet 105 is supplied downward from above, after the front and back surfaces of the molten sheet 105 are simultaneously contacted by the touch molding roll 101 and main molding roll 102, it is compressed by these rolls, and the compressed sheet 105 is wound on and fed from the main molding roll 102 to the following finish molding roll 103, thereby molding the sheet 105 having a specific thickness.
Also, according to the related-art sheet molding apparatus 120 shown in FIG. 11C, the sheet-shaped molding material 125 is supplied to the nip part 124 intervening between the first and second rolls 121 and 122 from the lateral direction.
In this apparatus, it is difficult to handle the sheet-shaped molding material 125 when it is fed to the nip part 124 intervening between the first and second rolls 121 and 122. Also, when molding a thin sheet, the sheet-shaped molding material 125 hangs down due to gravity so that cooling starts at one side of the molding material to be contacted first with the second roll 122, thereby disabling uniform cooling of both sides of the molding material. Thus, the apparatus 120 is incapable of molding a thin sheet which requires double-side simultaneous contact of the sheet-shaped molding material 125 with the first and second rolls 121 and 122.